Diseases caused by infectious agents and parasite infestations cause health problems and production losses in domestic animals but for many infectious agents no vaccine exists. Consequently, there are major research efforts worldwide to develop new vaccines which can protect against disease.
While some protective antigens from infectious agents and parasites have been identified, their successful use as vaccines requires the development of systems which can effectively deliver the antigen to the host. A variety of recombinant gene expression vectors derived principally from the pox virus family have been employed as these are generally of low pathogenicity. Expression of the foreign protein following infection by the recombinant viral vector may stimulate a protective immune response in the host.
However, no viral vector has all the attributes desirable for all situations. Some vectors are better suited to particular tasks than others because of their biological properties. For example, it has often proved difficult to stimulate an effective mucosal immune response which can protect against disease. In humans, adenoviruses have been given orally to vaccinate against respiratory disease (1). As this involves protection at mucosal surfaces adenoviruses clearly have potential in this regard. Human adenovirus vectors have also been used to deliver genes to muscle (2) and other tissues. Although adenoviruses do not generally integrate their DNA into the cellular genome, nevertheless, the DNA persists and long term protein expression is observed. Expression of an appropriate antigen from such cells can generate a systemic immune response which may be protective against the homologous disease causing agent.
Known adenovirus genomes are linear double-stranded DNA molecules which have an inverted terminal repeat sequence (ITR) at each end and a protein covalently bound to the 5′-terminal C residue (3). The genome sequence and structure has now been completely determined for human adenoviruses types 2, 5, 12 and 40 and partially for numerous others, including some animal isolates (see Genebank and EMBL Nucleic Acid databases). Human adenovirus type 2 was the first genome to be sequenced but broadly speaking its genome arrangement is conserved among other characterized adenoviruses i.e. early regions E1–E4 and the structural protein homologues can be recognized in similar locations in the genome. In particular, the E1A/E1B region is located at the left hand end of the genome and region E4 is always located at the right hand end of the genome. Early region E3 is always located between the genes for structural proteins pVIII and fiber, although its size and complexity varies between species e.g. from 3 kb with at least 10 open reading frames in human adenoviruses to approximately 0.7 kb with only two significant open reading frames in murine adenovirus (4, 5). E3 is a key region for the construction of recombinant viruses as it is non-essential for replication in vitro (6). The late, L region is expressed from the major late promoter, MLP and complex splicing generates families of mRNAs which code for most of the structural viral proteins. Proteins IVa2 and IX appear to have their own promoters.
Although there are some human viral vectors available for medical use there are few animal viral vectors suitable for use in veterinary applications. In order to obtain a more suitable animal viral vector the present inventors have purified an ovine adenovirus (OAV287) isolated from sheep in Western Australia. This ovine adenovirus is serologically related to bovine adenovirus type 7 but is genetically distinct from the bovine adenoviruses and other Australian ovine isolates, as shown by comparisons between the ovine and bovine adenoviruses, based on restriction enzyme profiles (8). The genome arrangement of the virus according to the present invention varies significantly from all other known adenoviruses. The adenoviral DNA molecule of the present invention is suitable for use in viral vectors capable of expressing a variety of polypeptides when used for veterinary applications.